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Review

Advances in Femtosecond Coherent Anti-Stokes Raman Scattering for Thermometry

by
Kaiyuan Song
1,
Mingze Xia
2,
Sheng Yun
1,
Yuan Zhang
1,3,
Sheng Zhang
3,
Hui Ge
1,
Yanyan Deng
1,
Meng Liu
1,
Wei Wang
1,
Longfei Zhao
1,
Yulei Wang
1,
Zhiwei Lv
1 and
Yuanqin Xia
1,3,*
1
Center for Advanced Laser Technology, Hebei Key Laboratory of Advanced Laser Technology and Equipment, Hebei University of Technology, Tianjin 300401, China
2
CNOOC Energy Technology and Services, Tianjin 300450, China
3
National Key Laboratory of Science and Technology on Tunable Laser, School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
*
Author to whom correspondence should be addressed.
Photonics 2024, 11(7), 622; https://doi.org/10.3390/photonics11070622
Submission received: 23 May 2024 / Revised: 20 June 2024 / Accepted: 27 June 2024 / Published: 28 June 2024
(This article belongs to the Special Issue Emerging Topics in High-Power Laser and Light–Matter Interactions)
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Abstract

:
The combustion process is complex and harsh, and the supersonic combustion flow field is also characterized by short duration and supersonic speed, which makes the real-time diagnostic technology for the transient environment extremely demanding. It is of great significance to realize high time-resolved accurate measurement of temperature, component concentration, and other parametric information of the combustion field to study the transient chemical reaction dynamics of the combustion field. Femtosecond CARS spectroscopy can effectively avoid the collision effect between particles in the measurement process and reduce the influence of the non-resonant background to improve the measurement accuracy and realize the time-resolved measurement on a millisecond scale. This paper introduces the development history of femtosecond CARS spectroscopy, points out its advantages and disadvantages, and looks forward to the future development trend to carry out high time-resolved measurements, establish a database of temperature changes in various complex combustion fields, and provide support for the study of engine mechanisms.

1. Introduction

Laser spectroscopy is the main non-contact method in high-temperature combustion temperature measurement [1,2]. The most significant advantage of this approach is that it neither interferes with the combustion field being measured nor is it affected by the combustion flow field [3,4]. Currently, high-temperature turbulent flame temperature measurements are conducted using infrared laser-absorption spectroscopy sensors [5,6], which provide temperature measurements with response times on the order of microseconds to milliseconds and a temperature measurement range of 400 K–3000 K. However, the limited spatial resolution and low temporal resolution of this method have led to an increase in the uncertainty in the high-temperature turbulent flame temperature measurements [7]. In addition, laser-induced fluorescence [8,9,10], laser-induced thermal grating spectroscopy [11], and tunable diode laser absorption spectroscopy [12] can also be used, but these methods also have limitations.
The femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopic technique has received much attention due to its non-interference characteristics, high accuracy [13], and high time resolution [14,15,16]. Although the CARS spectroscopic combustion field temperature measurement technique based on nanosecond laser sources is well established and widely used [17,18,19], it is nevertheless subject to the zero point non-resonant signal and collision effect due to the time scale of nanosecond, which in turn reduces the measurement accuracy and sensitivity of CARS spectroscopy technology [20]. Furthermore, the repetition frequency of nanosecond lasers is typically in the range of 10–20 Hz, which allows for only a few tens of temperature measurements per second. This limitation makes it challenging to apply the technique to transient temperature measurements in high-temperature and high-pressure turbulent combustion fields.
With the development of laser technology, ultrashort pulsed lasers have shown great potential in scientific research and industrial applications. Compared with traditional lasers, ultrashort pulsed lasers have pulse widths of femtoseconds or even attoseconds [21], peak powers of more than GW, and good temporal and spatial coherence. These characteristics make ultrashort pulsed lasers show unique advantages and broad prospects in many fields. In material processing, the ultra-high peak power and transient heating effect of ultrashort pulsed lasers can be used for precision micro–nano-manufacturing, and new component preparation can also be realized through cold material processing [22]. In the biomedical field, the low thermal effect and high spatial resolution of ultrashort pulsed lasers make them an effective tool for non-destructive biphotonic imaging and precision surgery [23]. In addition, ultrashort pulsed lasers show unique technical advantages and application prospects in high-precision probing, precision grating inscription, and carrier dynamics [24,25]. Especially, in combustion temperature measurement, the combination of CARS with femtosecond lasers enables the acquisition of thousands of temperature measurements per second, thereby making it possible to carry out real-time temperature measurements of transient combustion processes, such as the working cycle of internal combustion engines and jet flames. In addition, some scholars have also studied the nanosecond and femtosecond hybrid CARS techniques and have achieved some results [26,27,28].
Approximately 90 percent of the world’s energy supply today is generated by combustion. In the combustion field, the temperature of the flame affects the pathway and concentration of the chain reaction of various components in the field, and obtaining the temperature information of the combustion field can provide a significant basis for improving the efficiency of fuel combustion and the design of combustion devices. As a technology that can achieve transient temperature diagnosis in complex combustion scenarios, femtosecond CARS technology is necessary to summarize and prospect the research progress of this technology. In this paper, the research progress on applying femtosecond CARS temperature measurement technology in various combustion fields is reviewed and expected to provide a reference for the measurement method of high-temperature turbulent flame temperature.

2. Ultrashort Pulse CARS Spectroscopy Temperature Measurement

Femtosecond CARS spectroscopic technology is a non-contact method for measuring temperature based on the rotation spectrum of gas molecules without labeling or staining. In 1965, Make and Terhune [29] used a ruby laser to emit light, which produced an excited Raman signal as Stokes light in a benzene material. Together with the ruby laser as the pump light, this converged in the sample under test with a specific phase match, and they discovered a new four-wave mixed frequency signal, which they named CARS, and it was first used for gas-phase concentration measurements by Regnier [30] in 1974. In 1975, Moya [31] made the first temperature measurement using CARS signals of H2 molecules. CARS spectra reflect the distribution of particles in a gas at various energy levels, and information such as the temperature, concentration, and pressure of the gas can be obtained using the spectra. Furthermore, femtosecond CARS [32] typically implies that the laser pulse itself is shorter than the characteristic collision time scale of the objectives of the test. Thereby, measurements are hardly affected by collisional-quenching effects, and femtosecond CARS spectroscopy enables the direct measurement of turbulent combustion flames with time scales between 1 and 100 μs [33,34]. Ultrafast CARS was first used for gas-phase temperature measurements by Lang et al. in 1999 [35].
Figure 1 illustrates the energy level transition in the CARS generation process. Pump light and Stokes light reach the probe sample simultaneously, exciting the particle from the ground state to the virtual state due to the pump light. Excited Raman scattering occurs under the influence of Stokes light, causing the particle to transition from the virtual state to the first excited state. In the presence of probe light, the particle transitions from the first excited state to a higher virtual state and then returns to the ground state, producing a CARS signal. According to the different detection targets, the appropriate frequency, the excitation of the corresponding Raman vibration, the CARS signal light, and the three beams of incident light frequency are used to meet the following relationship: ω4 = ω1 − ω2 + ω3.
In addition to the aforementioned energy conservation, momentum conservation is also satisfied, expressed as k4 = k1 − k2 + k3, where k1, k2, k3, and k4 are the wave vectors corresponding to the pump light, Stokes light, probe light, and CARS signals, respectively. In 1978, Eckbreth [36] proposed the well-known BOX-CARS phase matching technique with Shirley et al. Subsequently, Shirley et al. [37,38,39] enhanced the BOX-CARS phase matching technique and proposed the folded BOX-CARS technique, as illustrated in Figure 2. This method of phase matching allows the CARS signal to be separated from the direction of the incident light in three-dimensional space, effectively preventing the incident light from interfering with the CARS signal measurement and further improving the spatial resolution of the CARS technique.
In recent years, with the rapid development of femtosecond laser technology and the maturity of high-power femtosecond laser products, people have gradually paid more and more attention to the study of femtosecond CARS spectroscopic temperature measurement technology [40,41,42,43,44,45,46], which has been applied in complex combustion environments, such as restricted spaces [47], turbulent flames [3], and other scenarios. After obtaining CARS spectra, the time and frequency domain expressions of the CARS signal intensity and the frequency domain electric field model can be simulated using MATLAB R2018a software, and the characteristics of the CPP-fs-CARS spectrum can be simulated and analyzed by debugging the specific parameter values in the equations to adjust the degree of chirp, resonance/non-resonance ratio, spot size, etc., and to build up the best theoretical model at room temperature. After determining the specific values of other parameters, it is only necessary to change to different temperatures in the theoretical spectra to build a library of theoretical models of CARS spectra at various temperatures. The experimentally collected spectrograms at high temperatures are compared with the established optimal theoretical spectrograms at different temperatures to extract the high-temperature flame temperature. The femtosecond CARS spectroscopic thermometry technique includes three variants of the techniques in experiments, namely, time-resolved femtosecond CARS spectroscopic thermometry, chirped probe pulse femtosecond CARS spectroscopic thermometry, and hybrid femtosecond/picosecond CARS spectroscopic thermometry. These are described in the following sections.

2.1. Femtosecond Time-Resolved CARS Spectroscopy

In CARS spectroscopy, in addition to the CARS signal, various background noises, such as scattered light, autofluorescence of the sample, and non-resonant signals, are also detected. Among them, the non-resonant background noise comes from the non-resonant part of the third-order nonlinear polarization rate [48]. Since the wavelength of the non-resonant background noise is very close to that of the CARS signal, it is difficult to eliminate it by filtering methods, and these noises affect the spectral resolution and detection sensitivity of CARS spectra. Therefore, several methods have been proposed to suppress the non-resonant background noise, such as the polarization-sensitive detection method [49], the back detection method [50], the phase shaping method [51], and the time-resolved method [52,53,54]. The time-resolved CARS (T-CARS) method can eliminate the non-resonant background noise by taking advantage of the different dephasing times of the resonant signal and the non-resonant background noise [55].
Femtosecond CARS is mainly based on the decay of Raman coherence after initial excitation of the pump and Stokes pulses to temperature measurements [56,57,58,59,60]. The reason for the Raman coherence decay is that the individual Raman transitions are slightly different in oscillation frequency and no longer interfere constructively [61,62]. Because of the number of Raman transitions with significant population increases with temperature, temperature measurements can be carried out this way. Time-resolved CARS spectroscopy is based on the conventional CARS spectroscopic temperature measurement technique, with the addition of a high-precision linear stage, using the frequency difference between the pump pulse and the Stokes pulse to stimulate the Raman mode of the sample and detecting it with the probe light. By continuously varying the time the probe light reaches the sample to be measured, it is possible to obtain a series of CARS spectra on the femtosecond time scale. This allows the time-resolved CARS signal to be recorded, which provides insight into the trend of signal intensity over time, and this time-dependent signal strength can be fitted by building a theoretical model to obtain the temperature information of the target sample. A typical experimental setup is shown in Figure 3, and Table 1 demonstrates the research progress of time-resolved CARS thermometry in recent years.
The idea of using femtosecond time-resolved CARS to measure combustion field temperatures was first proposed and demonstrated to be feasible in 1999 by Motzkus et al. [35] at the Max-Planck-Gesellschaft. Then, they also achieved temperature measurements from 300 K to 1100 K by time-resolved spectroscopy of H2 with a measurement error of 30 K [58,66]. They also measured the time-resolved spectra of signal changes at different pressures [67].
In 2006, Lucht et al. [62] at Purdue University obtained CARS signals for gas samples using probe pulses with different delay times and measured a heated gas cell in the temperature range of 300 K–940 K. In 2008, they extended the temperature range to within 1500 K to 2500 K [59] with an accuracy of ±40 K and a precision of ±50 K. In 2009, they used time-resolved spectroscopy followed by Fourier transformation to improve the accuracy and precision of temperature measurement from 300 K–2400 K to 1–6% and 1.5–3%, respectively [64].
In 2012, our team [63] conducted a study to measure the temperature of a methane/O2/N2 premixed flame at atmospheric pressure using the femtosecond CARS technique. A 40 fs laser pulse was employed to excite the rotational spectrum of N2, with the CARS signal being measured within a few picoseconds of the initial coherent excitation. The flame temperatures were measured at 300 K–1325 K. The results were in good agreement with theoretical calculations and exhibited good repeatability.
However, femtosecond time-resolved CARS has some inherent drawbacks. Firstly, the traditional time-resolved CARS spectroscopy system is complex and expensive [68,69], and secondly, the measurement process needs to take points several times, which makes the technique unable to achieve the measurement of transient processes. Furthermore, for samples containing multiple components to be measured, the center wavelength of the laser needs to be changed continuously to obtain the CARS spectral information of each component in the sample, which is not conducive to practical applications [70,71,72]. Due to these limitations, this technique has not been studied much in temperature measurement, while femtosecond single-pulse CARS thermometry has received attention, which mainly includes femtosecond/picosecond hybrid CARS spectroscopy and chirped probe pulse femtosecond CARS spectroscopy, which allows for transient measurements of the combustion field with a high time resolution.
It is worth noting that there are also scholars who are looking for some solutions to the above shortcomings. In 2024, Song [65] introduced additional probe pulses into the conventional time-resolved CARS system [52], and the dual-probe scheme allows for obtaining more spectral information in a single measurement [9,73]. In this dual-probe scheme, the pump pulse and the Stokes pulse interact with the N2 molecule simultaneously, while the two probe pulses are positively delayed by 3.44 ps and 2.88 ps, respectively, and the temperature of the system can be determined by comparing the intensity ratios of the different CARS signals generated. Thus, the stepwise scan on the relaxation of the vibrational coherence is no longer required when performing temperature measurements. Two probe beams of different wavelengths will produce two CARS signals of different wavelengths that can be distinguished in the same spectrum so that single-shot measurement can be realized in principle [66]. However, at present, the method only has good precision and accuracy in monitoring flame temperatures below 2000 K. This is mainly due to the large fluctuations in CARS intensity above 2000 K, which can directly affect the measurement accuracy. Overall, this method is a potential candidate for accurate monitoring of real-time temperatures in turbulent combustion.

2.2. Femtosecond Single-Pulse CARS Technology

Typical femtosecond laser spectra have a spectral width of about a few tens of nanometers (about 200 cm−1). When the incident pump light and Stokes light are both broad spectrum femtosecond pulses, some frequencies in the pump pulse may meet the same frequency difference with several frequencies in the Stokes pulse, and multiple pairs of Raman transitions at the same frequency can be excited at the same time. At the same time, the fixed-frequency component of the pump light may also meet the requirement of coherent excitation with many different frequencies in the broad spectrum of Stokes and can scan Stokes light frequencies without adjusting the time delay to realize the coherent excitation of Raman transitions of molecules at different vibrational/rotational energy levels, which is the femtosecond single-pulse CARS thermometry technology. Femtosecond single-pulse CARS techniques can be classified into two categories: chirped probe pulse CARS, which uses a short chirped pulse as the excitation source, and hybrid femtosecond/picosecond CARS, which uses femtosecond main pulses and picosecond pulses. These two schemes enable high time resolution temperature measurements and overcome the technical constraints of conventional CARS systems. They are powerful tools for studying combustion dynamics.

2.2.1. Femtosecond Chirped Probe Pulse CARS

Femtosecond chirped probe pulse CARS (CPP-fs-CARS) is a novel single-pulse CARS spectrometry technology. Based on the common femtosecond CARS, CPP-fs-CARS utilizes two synchronously locked femtosecond lasers. One laser generates the main pulse with stable intensity, while the other generates a short chirped probe pulse. The chirped pulses are synchronized in time with the main pulse, and the time domain stretching and frequency domain chirping of the probe pulse are achieved by adding dispersive media to the probe optical path, which stretches the probe pulse width from femtoseconds to picoseconds while the different frequency components are separated on the time axis. The Raman coherent wave packet, driven by a single femtosecond pump pulse, interacts with different frequency components of the probe pulse at different times, resulting in the corresponding CARS response in the frequency domain. Unlike conventional time-resolved CARS, which uses delay line scanning, CPP-fs-CARS uses a single pulse to directly acquire time series data, greatly simplifying the system. Figure 4 shows a typical experimental setup.
The CPP-fs-CARS technique was initially proposed in 2002 by Lang and Motzkus et al. at the Max-Planck-Gesellschaft [75]. In their experiments, they introduced chirp using 6 cm SF-10 glass rods, which broadened the 100 fs probe pulse to 500 fs. The CARS signal also had a chirp effect, enabling the mapping of the time curve to the frequency domain. The results indicate that the transient shape agrees well with multiple measurements in the region where the delay time exceeds the duration of the modulated pulse, with a small phase shift. By applying this technique to a single temperature measurement of H2, a temperature measurement range of 300 K to 1100 K was achieved with an error of 30 K, which is consistent with the theoretical fit. Table 2 demonstrates the research progress of CPP-fs-CARS thermometry in recent years.
Roy and Lucht at Purdue University have carried out extensive thermometry work using CPP-fs-CARS. In 2009, they determined the temperature from fitting a single spectrum of the CARS signal by varying the delay time of the probe pulse for the pumping and Stokes excitation (~2 ps) [64]. The accuracy and precision of the method were 1–6% and 1.5–3% in the temperature range of 300 K–2400 K, respectively. A single temperature measurement of gas-phase N2 was achieved for the first time at a rate of 1 kHz by fitting the experimental results to the theoretical calculations by the least-squares method. Compared to H2, N2 is more suitable for temperature measurement of combustion gas streams. This work thus extends the application of the technique with the promise of application to transient and turbulent systems but has not yet applied it to actual combustion fluids and has only validated it under ideal conditions.
To validate the technique in a practical situation, in 2011, Lucht and Richardson [76] performed 1000 Hz temperature measurements on a flame driven by a piston and a turbulent methane air flame. A theoretical model of CPP-fs-CARS considering the parameters of the pump pulse, the Stokes pulse, and the probe pulse led to the development of resonant and non-resonant polarization contributions. The standard deviation of the technique in gas cells and laminar flames is less than 2% of the mean temperature. Periodic fluctuations of 10 Hz were accurately captured in piston-driven flames, validating the ability of the technique to detect transient phenomena. Then, they discussed suppressing the non-resonant background by polarization techniques for CPP-fs-CARS measurements [80], providing a methodology for the further development of high-fidelity CPP-fs-CARS models.
The vibrational modes of methane are easier to detect because the methane scattering cross-section is eight times larger than the scattering cross-section of N2 [81]. In 2014, Dennis modified the objective function of the spectral fitting algorithm to calculate the energy levels of N2 on top of Richardson’s work so that the temperatures of N2 and methane could be fitted simultaneously. Subsequently, Dennis used the modified code to perform 5 kHz femtosecond CARS temperature measurements in an out-of-core jet diffusion flame [82], a swirl-stabilized combustor [83], and a gas turbine model combustor [84], respectively.
Another validation was carried out in 2016 by Dennis and Lucht [77] in a more practically relevant high-temperature turbulent flame. Measurements were made at 73 points within the gas turbine model combustion in different steady-state environments, and a CARS signal sufficient for analysis was obtained for almost every laser hit. The spatial resolution of the single-laser temperature measurements was about 600 µm, the precision was about ±2%, the accuracy was about ±3%, and the dynamic range was sufficient for temperature measurements from 300 K to 2200 K. Lucht subsequently improved the spectral fitting method based on the statistical method of maximum likelihood to achieve a temperature accuracy and precision of 2.7% and ±3.5% for flame temperature and 9.9% and ±6.1% at room temperature [78] and devised a scientific method to evaluate the system’s performance. Results from multiple sets of laser parameters are combined to generate an error-weighted temperature from the top-performing calibrations. This method provides a basis for the further application of the technique to actual spray combustion processes.
Thomas at Purdue University and Lowe at the University of Sydney also measured temperature in turbulent spray flames using the CPP-fs-CARS technique with a repetition rate of 5 kHz. In 2019, they performed temperature measurements in spray turbulent flames with ethanol droplets of varying concentrations, achieving an integration time of 3 ps and a spatial resolution of ~800 µm along the direction of beam propagation and lateral dimensions of ~60 µm of the spatial resolution [79]. The accuracy of the technique was verified with a relative accuracy and precision of 2.8% and ±3.4%, respectively, at peak flame temperatures of up to 2512 K. The results show the accuracy of the technique. The spray flame data were processed to obtain the average axial and radial temperature distributions in both the dilute and dense spray flames. The observed temperature range was between 1000 K and 2000 K, as shown in Figure 5.
They then went on to measure the temperature fields of ethanol and acetone spray flames separately using this technique [85]. Flames with different spray densities were obtained by varying the nozzle recession length (0 mm–80 mm), as shown in Figure 6, where the nozzle recession length significantly affects the flame structure at x/D > 10. Temperature probability density function analysis showed that the dense spray inhibited the inhalation of hot flying bodies into the spray envelope, reducing the vaporization and combustion speed rates. With a temperature measurement range of about 300 K–2500 K and a temperature measurement accuracy of ±2%, this method was used to measure, for the first time, the transient temperature fields of ethanol and acetone spray flames under different spray density conditions using a high-speed CPP-fs-CARS.
With the increase in carbon-containing fuels, researchers gradually began CARS temperature measurements using carbon dioxide (CO2) as a probe molecule. Lucht developed the phenomenological model of CO2/O2 femtosecond CARS in 2021 [13]. Validation experiments in different CO2/O2 gas mixtures demonstrated that O2 interference could be eliminated at a probe pulse delay of approximately 9 ps. Flame temperature tests were conducted on a Hencken burner, and the results showed that CO2 femtosecond CARS can accurately measure temperatures under 1200 K, with up to 1% accuracy under 1100 K. Additionally, precise measurements can be obtained with a single set of laser parameters by increasing the probe pulse delay of the CO2 femtosecond CARS. The CO2 femtosecond CARS technique offers several advantages. Firstly, it is not affected by interference from O2 and non-resonant backgrounds. Secondly, it is more sensitive to flames at lower temperatures (<1000 K).
Lucht and Chang [14] recently demonstrated hydrogen CPP-fs-CARS temperature measurements in a high-pressure model rocket motor combustion chamber. For the Hencken burner flame, time-averaged spectra were taken at 144 Hz for an improved SNR. Around 2000 spectra were averaged and fit using the fitting algorithm. Figure 7 below shows an excellent fit between the experimental and theoretical spectra. They also acquired CPP-fs-CARS spectra along the axial direction of the combustion chamber under gaseous H2/O2 fuel conditions at pressures of up to 7 MPa and an equivalence ratio of 3.0. This was the first time that the CPP-fs-CARS technique was applied to high-pressure rocket motor experiments that yielded transient temperature field information with a high time resolution, demonstrating the feasibility of using this technique to measure high-pressure rocket motor temperatures.
The chirped pulses in the CPP-fs-CARS technique are obtained based on the dispersion effect extension, and the spectrum of the signal produced by this method has the inherent limitation of a low resolution. In addition, their complex spectra increase the difficulty of the fitting process. To overcome this limitation, the researchers proposed a hybrid femtosecond/picosecond CARS technique using a pulse shaper to generate narrow linewidth picosecond probe pulses. By independently controlling the time domain and frequency domain parameters of the probe pulse, this method realizes the organic combination of the high temporal resolution and high spectral resolution, which compensates for the inherent limitations of CPP-fs-CARS and further extends the application range of the single-pulse CARS technique.

2.2.2. Hybrid Femtosecond/Picosecond CARS Spectroscopy for Temperature Measurement

The hybrid femtosecond/picosecond (fs/ps) CARS technique employs a pulse shaper to generate picosecond probe pulses with narrower linewidths in the frequency domain, which have a significantly higher spectral resolution than chirped pulses obtained using dispersion effect expansion [86,87]. As a result, the hybrid fs/ps CARS system can measure purely rotationally dynamic CARS processes, whereas the broadband CPP-fs-CARS system struggles to achieve the same level of high-precision rotational spectral detection. In addition, each peak in the hybrid fs/ps CARS spectra directly corresponds to a specific rotational Raman jump, which more intuitively reflects the rotational temperature information. Compared with CPP-fs-CARS, which requires complex iterations to fit the entire spectral curve, the hybrid fs/ps CARS spectra are simpler to fit and less computationally intensive [88]. It can be seen that the high spectrally resolved probe pulses generated by pulse shaping are the keys to achieving high temporal and spectral resolutions in hybrid fs/ps CARS. This novel single-pulse excitation strategy refines the advantages of the femtosecond CARS spectroscopy technique in terms of accuracy and ease of use, and a typical experimental setup is shown in Figure 8.
Hybrid fs/ps CARS technology is also divided into two main categories, namely, purely vibrational hybrid fs/ps CARS and purely rotational hybrid fs/ps CARS. Purely vibrational hybrid fs/ps CARS technology uses femtosecond pump pulses and picosecond probe pulses of similar center wavelengths to excite vibrational excitations of the gas, generating the corresponding vibrational CARS signals. Unlike conventional CARS systems that scan the laser wavelength, a full spectrum of vibrational radiation can be obtained by independently controlling the parameters of the femtosecond and picosecond laser pulses. In particular, adjusting the time delay of the picosecond probe pulse controls the temporal resolution of the CARS process, and varying the relative intensity of the picosecond pulse controls the suppression of the non-resonant background. Vibrational CARS spectral thermometry shows good accuracy in the temperature range above 1000 K. Pure rotation hybrid fs/ps CARS, on the other hand, uses femtosecond and picosecond pulses with large wavelength differences to excite different rotational transitions, and this technique can distinguish the transitions between different rotational quantum numbers and provide high-resolution rotational Raman spectra, thus accurately measuring the rotational temperature of the gas, which shows high sensitivity at lower temperatures.
Hybrid fs/ps CARS spectroscopy was proposed by Prince and Chakraborty et al. [90] in 2006. Extensive research in hybrid fs/ps CARS temperature measurement has been performed by Miller and Meyer. In 2010, they performed high-speed temperature measurements in a high-temperature gas-phase system [91]. Using a 100 fs broadband pump and Stokes light to excite the vibrational energy levels of N2, the Raman resonance response was probed with a frequency-narrowed 2.5 ps probe pulse (delayed by 2.36 ps from the pump and Stokes pulses), which reduced the non-resonant background by two orders of magnitude. Experimentally obtained spectra were performed in steady-state and pulsating H2–air flames at a rate of 500 Hz, exhibiting a temperature precision of 2.2 percent and an accuracy of 3.3 percent at 2400 K, enabling fast and precise measurements of high-temperature gas-phase temperatures. Table 3 demonstrates the research progress of hybrid femtosecond/picosecond CARS thermometry in recent years.
Miller then used the same broadband pump and Stokes pulses to excite the molecular rotational energy levels. Raman coherence was then probed using a frequency-narrowed 8.4 ps probe pulse after a 13.5 ps time delay to eliminate non-resonant background interference [92]. The technique enables the direct measurement of the collisional out-of-phase rate for each rotational energy level (J-value) within the range of 13.5 ps–100 ps. The results demonstrate that the temperature measurement error is less than 1% at atmospheric pressure when the probe pulse delay is less than 30 ps, indicating no collisional effect. Frequency and time domain model calculations demonstrate that, under current conditions, the fs/ps CARS achieves a best-fit temperature accuracy of up to 1% within the range of 306 K–700 K at a 13.5 ps time delay, as demonstrated in Figure 9. This method allows for fast and accurate gas temperature measurements.
In 2014, Meyer [73] developed a dual-pump hybrid fs/ps CARS system capable of exciting both rotation–vibrational and pure rotation transitions of multiple molecules simultaneously. The technique utilizes two beams of 100 fs femtosecond pump pulses at 660 nm and 798 nm to simultaneously excite N2/CH4 vibrational and N2/O2/H2 rotational Raman coherence. A common beam of 798 nm femtosecond Stokes pulse is used, with a narrow-band 798 nm picosecond probe pulse detecting all the Raman resonance responses after a certain time delay to suppress non-resonant background and collisional effects. The system achieves quantitative detection of four key combustion species, namely, N2, O2, CH4, and H2, as well as accurate temperature measurement over a wide temperature range from 298 K to 2300 K through broadband excitation and multiplexing of multiple molecular transitions, and the temperature measurements were precise, with an accuracy of within 2% of the theoretical predictions for equilibrium.
In 2015, Kearney [41] employed second harmonic bandwidth compression (SHBC) to produce high-energy sub-10 ps probe pulses for collision-free and non-resonant background measurements at kilohertz rates. For a peak SNR of more than 60, attained for temperatures of up to T = 1660 K in the C2H4/air flame of the McKenna burn-errand T = 1550 K in the near-adiabatic H2/air flame stabilized on the Hencken burner, temperature-measurement precision is 1–1.5%, which is a factor of 3–4 better than the best nanosecond rotational CARS measurements [95,96] at atmospheric pressure. This temperature measurement precision is comparable to the best results measured using femtosecond vibrational CARS schemes at flame temperatures above 2000 K [74,92] and meets or exceeds the accuracy shown using rotating fs/ps CARS in low-temperature environments in air [97] and pure N2 [98,99].
To predict and interpret the effect of spectrally significant modulation of the probe pulses on the hybrid fs/ps pure rotation CARS results, Yang of the Chinese Academy of Engineering Physics and Escofet-Martin at the University of Edinburgh [100] used a home-built SHBC in 2017 to generate picosecond pulses with different spectrally significant modulations. The simulation results with the standard model and modified model are shown in Figure 10. For the fitting of every probe delay, the residuals with the chirped model are smaller than those with the original model. However, the probe pulse chirp affects the sum of the absolute residuals, and it takes a certain amount of temperature bias to match the valley values of non-chirp summations of absolute residuals.
In 2020, Escofet-Martin [93] introduced another novel dual-detection one-dimensional fs/ps hybrid rotation CARS spectroscopy technique for simultaneous single-shot temperature, pressure, and O2/N2 concentration ratio measurements in the gas phase. The technique was used to extract the time and frequency domain information simultaneously and to make accurate and precise pressure measurements based on the strong correlation between the CARS signal and the pressure in the time domain and temperature measurements in the frequency domain. Highly accurate (<1%) pressure measurements in the pressure range of 1 bar–1.5 bar and the temperature range of 280 K–310 K, with spatiotemporal accuracies of 0.62% for temperature and 0.42% for pressure and the ability to characterize pressure gradient variations of 0.04 bar/mm, were achieved, demonstrating the promise of the novel technique for application in high-pressure, high-temperature, and high-concentration gradient settings.
In the same year, Li [89] at Tsinghua University proposed a hybrid fs/ps CARS thermometry method using an optimized 10 ps-35 ps probe time delay. The optimal time delay, corresponding to the maximum value of the differential at each temperature, was found by calculating the differential of the spectra at neighboring temperatures. The CARS spectrum exhibits another distinct peak when using the optimal time delay. This facilitates the fitting process and avoids temperature errors caused by a slight drift in the probe center wave number. The ability to maximize temperature sensitivity in the range of 300 K to 2500 K shows great potential in applications requiring high sensitivity. Then, they proposed a quasi-common-path SHBC method [101] to achieve spectral filtering and introduce linear dispersion through the same 4f shape filter. This method resulted in a high-energy narrow-band picosecond pulse with a center wavelength of 401.5 nm, a bandwidth of about 7 cm−1, and a pulse energy of 240 µJ from a 35 fs broadband pulse. The given pulse served as a probe pulse for a hybrid fs/ps vibrating CARS system. This system allowed for single-pulse temperature measurements of methane/air flames at a rate of 1000 frames per second. The temperature uncertainty of the single-pulse measurements was less than 1%, with an accuracy of 2% at the optimal probe pulse delay time for a 1790 K flame. The method demonstrates excellent accuracy and sensitivity in harsh environments where fast temperature measurements are required.
CARS has been extensively used in harsh combustion environments, such as high-pressure gas turbines and rocket combustors [102,103,104,105,106]. In 2023, Dedic [94] used the fs/ps CARS technique to measure the temperature inside the University of Virginia Supersonic Combustion Facility (UVASCF) [107,108]. Previous CARS measurements in the UVASCF have used nanosecond laser excitation [109,110]. The UVASCF requires multiple viewports in the combustion chamber wall for spectral measurements using laser light. However, the interaction of the femtosecond laser with these glass windows produces a supercontinuum spectrum that affects the measurement accuracy. It reduced the generation of supercontinuum spectra by controlling the energy density of the femtosecond laser on the windows. Additionally, the optical path was optimized to improve the laser’s focusing quality at the measurement point, resulting in a CARS signal of sufficient intensity. The CARS system’s performance was tested in an H2/air flame, and the results indicate that the simulated spectra are in good agreement with the single experiment spectra at 2300 K. This provides a foundation for subsequent studies of the combustion state of a dual-mode ram engine in this high-speed airflow test bed.

3. Conclusions

Femtosecond CARS spectroscopy techniques play an increasingly important role in non-contact optical temperature measurement. These techniques offer ultra-high temporal resolution for studying transient processes and non-equilibrium temperature measurements. Femtosecond CARS spectroscopy is also useful for Raman spectroscopy measurements at low wave numbers, providing information on rotational and low-frequency vibrational modes. Future developments aim to enhance spatial and temporal resolution, improve the precision and accuracy of temperature measurements, and achieve three-dimensional scanning imaging. Progress is expected in probe development, expanding measurement range, and system integration. Collaboration between researchers and industries is crucial for establishing a non-contact temperature measurement platform, which will provide more accurate technical support for complex high-temperature turbulent combustion field temperature measurements.

Author Contributions

K.S.: conceptualization, investigation, and original draft; M.X.: formal analysis; S.Y.: supervision and editing; Y.Z.: supervision and editing; S.Z.: editing; H.G.: project administration; Y.D.: project administration; M.L.: project administration; W.W.: project administration; L.Z.: project administration; Y.W.: project administration; Z.L.: project administration; Y.X.: supervision and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of China (62375074, 62075056); the Natural Science Foundation of Hebei Province in China (F2023202082, F2022202035); and the Interdisciplinary postgraduate Training Program of Hebei University of Technology (HEBUT-Y-XKJC-2022120).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

Author M.X. is employed by the company CNOOC Energy Technology and Services that this research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The other authors declare no conflicts of interest.

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Photonics 11 00622 g001
Figure 1. Schematic of molecular states and the CARS signal generation process. v,J and v’,J’ are different rovibrational states, and ωR is the Raman resonance frequency of the medium molecule [32].
Figure 1. Schematic of molecular states and the CARS signal generation process. v,J and v’,J’ are different rovibrational states, and ωR is the Raman resonance frequency of the medium molecule [32].
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Figure 2. Folded BOX-CARS phase matching method.
Figure 2. Folded BOX-CARS phase matching method.
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Figure 3. Schematic diagram of the time-resolved femtosecond CARS experimental setup [63].
Figure 3. Schematic diagram of the time-resolved femtosecond CARS experimental setup [63].
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Figure 4. Schematic diagram of the single-laser-shot CPP-fs-CARS measurements [74].
Figure 4. Schematic diagram of the single-laser-shot CPP-fs-CARS measurements [74].
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Figure 5. The radial profile of the mean temperature of the dilution spray (N-EF8-80) and the dense spray (N-EF8-25) at x/D = 0.4–20; x/D represents the length ratio to total length, the inverted triangles represent fitting errors, and circles represent temperature [79].
Figure 5. The radial profile of the mean temperature of the dilution spray (N-EF8-80) and the dense spray (N-EF8-25) at x/D = 0.4–20; x/D represents the length ratio to total length, the inverted triangles represent fitting errors, and circles represent temperature [79].
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Figure 6. Long-exposure (1 s) images of samples with acetone flame at different recess lengths; x/D represents the length ratio to total length, with the pilot stream in channel 1, the main air jet stream in channel 2, and the liquid fuel stream in channel 3 [85].
Figure 6. Long-exposure (1 s) images of samples with acetone flame at different recess lengths; x/D represents the length ratio to total length, with the pilot stream in channel 1, the main air jet stream in channel 2, and the liquid fuel stream in channel 3 [85].
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Figure 7. H2 CPP-fs-CARS spectra in a Hencken burner flame with theoretical fit at an adiabatic flame temperature of 2129 K and 0.1 MPa (1 atm), where the blue lines represent the residuals, meaning the difference between the actual observations and the model predictions [14].
Figure 7. H2 CPP-fs-CARS spectra in a Hencken burner flame with theoretical fit at an adiabatic flame temperature of 2129 K and 0.1 MPa (1 atm), where the blue lines represent the residuals, meaning the difference between the actual observations and the model predictions [14].
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Figure 8. Hybrid femtosecond/picosecond CARS experimental setup [89]. OPA: optic parametric amplifier; BS1-2: beam splitter; L1-2: lens; M16: mirror; TDL1-2: tunable delay line; G1-2: diffraction grating; P1-2: prism; CL1-2: compensation lens; EMCCD: electron-multiplying charged coupled device.
Figure 8. Hybrid femtosecond/picosecond CARS experimental setup [89]. OPA: optic parametric amplifier; BS1-2: beam splitter; L1-2: lens; M16: mirror; TDL1-2: tunable delay line; G1-2: diffraction grating; P1-2: prism; CL1-2: compensation lens; EMCCD: electron-multiplying charged coupled device.
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Figure 9. Single-shot pure rotational hybrid fs/ps CARS spectra of N2 at (a) 306 K, (b) 500 K, and (c) 700 K, where open symbols represent experimental data while solid lines represent best-fit simulations [92].
Figure 9. Single-shot pure rotational hybrid fs/ps CARS spectra of N2 at (a) 306 K, (b) 500 K, and (c) 700 K, where open symbols represent experimental data while solid lines represent best-fit simulations [92].
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Figure 10. Comparison between the fitting results of the modified model (with probe chirp) and the original model (without chirp). Two groups of coherent anti-Stokes Raman scattering spectra of N2 (black solid lines) with different probe chirps are fitted [100].
Figure 10. Comparison between the fitting results of the modified model (with probe chirp) and the original model (without chirp). Two groups of coherent anti-Stokes Raman scattering spectra of N2 (black solid lines) with different probe chirps are fitted [100].
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Table 1. Research progress of time-resolved CARS temperature measurement technology.
Table 1. Research progress of time-resolved CARS temperature measurement technology.
ResearcherYearTarget MoleculesApplicationTemperature/KAccuracyPrecision
Motzkus [35]1999H2Sealed-off quartz cell300–1100//
Lucht [59]2008H2Hencken burner1500–25001.6–2.7%2–3.3%
Lucht [64]2009H2Hencken burner300–24001–6%1.5–3%
Xia [63]2012N2Methane/O2/N2 flame300–1325//
Song [65]2024N2Swirl burner850–2000/~3.7%
Table 2. Research progress of CPP-fs-CARS temperature measurement technology.
Table 2. Research progress of CPP-fs-CARS temperature measurement technology.
ResearcherYearTarget MoleculesApplicationTemperature/KAccuracyPrecision
Lang [75]2002H2Combustion cells300–1100~2.7%/
Roy [64]2009N2Near-adiabatic flame300–24001–6%1.5–3%
Lucht [76]2011N2Hencken burner1790–1940~2%5%
Lucht [77]2016N2Dual-swirl gas turbine model combustor300–2200±3%±2%
Lucht [78]2017N2Hencken burner295–22952.7%±3.5%
Thomas [79]2019N2Turbulent spray flames25122.8%±3.4%
Lucht [13]2021CO2/N2Hencken burner295–1420N2: 1.1–8.9%
CO2: 0.6–5.3%		1.6% (>1200 K)
1.1–1.4% (<1200 K)
Chang [14]2023H2High-pressure rocket chamber2000–3000//
Table 3. Research progress of hybrid femtosecond/picosecond CARS temperature measurement technology.
Table 3. Research progress of hybrid femtosecond/picosecond CARS temperature measurement technology.
ResearcherYearTarget MoleculesApplicationTemperature/KAccuracyPrecision
Miller [91]2010N2Unsteady high-temperature flames2400 ~3.3%2.2%
Miller [92]2011N2The time delay is 13.5 ps to 30 ps306–700/1%
Miller [73]2014O2/H2/N2Adiabatic
H2–air Hencken burner flame		298–2300/RCARS: 5%
VCARS: 2%
Kearney [41]2015N2Near-adiabatic H2/air flames
Premixed C2H4/air flames		H2: 1550
C2H4: 1660		/1–1.5%
Escofet-Martin [93]2020N2Pressure (0.9–1.1 bar)280–310 0.62%0.42%
Li [89]2020N2Hencken burner21101.2%/
Kim [94]2023C2H4Supersonic combustion facility2294//
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Song, K.; Xia, M.; Yun, S.; Zhang, Y.; Zhang, S.; Ge, H.; Deng, Y.; Liu, M.; Wang, W.; Zhao, L.; et al. Advances in Femtosecond Coherent Anti-Stokes Raman Scattering for Thermometry. Photonics 2024, 11, 622. https://doi.org/10.3390/photonics11070622

AMA Style

Song K, Xia M, Yun S, Zhang Y, Zhang S, Ge H, Deng Y, Liu M, Wang W, Zhao L, et al. Advances in Femtosecond Coherent Anti-Stokes Raman Scattering for Thermometry. Photonics. 2024; 11(7):622. https://doi.org/10.3390/photonics11070622

Chicago/Turabian Style

Song, Kaiyuan, Mingze Xia, Sheng Yun, Yuan Zhang, Sheng Zhang, Hui Ge, Yanyan Deng, Meng Liu, Wei Wang, Longfei Zhao, and et al. 2024. "Advances in Femtosecond Coherent Anti-Stokes Raman Scattering for Thermometry" Photonics 11, no. 7: 622. https://doi.org/10.3390/photonics11070622

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